The susceptibility of the developing nervous system to environmental agents has been a major concern with regard to children's health issues. While current exposure levels to environmental agents does not represent an acute injury, disruption to the nervous system may be associated with either a structural alteration in the formation of the neural network and/or in nervous system functioning. The formation and interactions between the various cell types in the brain are critically timed events.Such windows of vulnerability is assummed to be a major component in the differential susceptibility of the developing organism to environmental insult. This project examines chemical induced perturbations during development of the nervous system as indicated by various alterations in the morphology and molecular profile in the developing brain and assoicated neurobehavioral outcome of such exposure. The specific projects under study include 1) Neurobehavioral effects of thimerosal administered to immature SJL autoimmune mice, replication and expansion of literature report including unbiased stereology of brain regions and mercury distribution. 2) Working with the Toxicology Operations Branch, we are actively establishing a mouse model to examine the neurodevelopmental toxicities of environmental chemicals that perturb thyroid hormone homeostatic maintenance during gestational and postnatal development. In addition to changes occurring during development, this project will also focus on questions related to either latent effects or effects on the aged brain as the result of early developmental exposure to environmental chemicals. For these studies we continue to use a number of methods to examine alterations in the developing nervous system following exposure to environmental agents including immunohistochemistry, molecular techniques to examine mRNA levels, as well as assessment of neurobehavioral functioning.